Mammalian target of rapamaycin (mTOR) is a 289 kD serine/threonine kinase identified as a target of rapamycin, a macrolide antimicrobial agent.
mTOR (also referred to as FRAP, RAPT1 or RAFT1) is expressed in almost all organs and tissues and is involved in the PI3K-Akt signaling system. mTOR forms mTORC1 and mTORC2 complexes together with adaptor proteins such as raptor and rictor, respectively, and transduces extracellular signals. mTORC1 activates translation of cancer-related proteins (such as cyclin D1, myc and HIF-1α) by phosphorylating its downstream targets such as S6K and 4EBP-1. On the other hand, mTORC2 is assumed to activate survival signals of cancer cells by phosphorylating its downstream target Ser473 of Akt.
Clinically, the mTOR signaling system has also been observed to be enhanced in many cancer types including renal cancer, osteosarcoma, lung cancer, ovarian cancer, prostate cancer, breast cancer, colon cancer and liver cancer.
Rapamycin, an mTOR inhibitor, binds intracellularly to FKBP12 (FK-506 binding protein) and forms a complex. Then rapamycin/FKBP12 complex binds to mTOR; mTOR kinase activity is assumed to be inhibited by this binding, and protein synthesis and cell proliferation are also inhibited as the results. Rapamycin has in fact been reported as having an antitumor effect in tumor patients. Currently, clinical trials have been performed for mTOR inhibitors including CCI-779, a rapamycin derivative.
As described above, it is thought that mTOR is an effective molecular target for cancer therapy, and that compounds having an effect of inhibiting mTOR kinase activity (hereinafter sometimes referred to as mTOR inhibitory activity) can be pharmacologically useful in cancer therapy, in particular, for cancer with enhanced mTOR signaling, e.g., cancer with LKB mutations or TSC2 mutations, or cancer with inactivated PTEN (Non-patent References 1 to 3).
Compounds such as pyridopyrimidine derivatives and imidazopyrazine derivatives have been known to inhibit mTOR activity.